Here is my theory (it has ever so be proven numerous times!) When it comes to MP games, the person wants to be with other people (This is the SAMP version) and they want to be with more people, so they hop onto a almost full server. Now when they join, they will attract more people (for instance, a full server, you will have a lot of people waiting, and more and more people want into this "Full server". Attracting more people as it goes.
Simplified - Full Server, Attracts more People, as a Server with alot of slots open, attracts less people.
The G-Mod (Any MP Half Life game) has a wierd theory, close but far.
Lets lower the playerslots, but hosted again, and resume the old days.